1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing devices using the exposure apparatus, the exposure apparatus being used in a photolithography process included in processes of manufacturing devices, e.g., semiconductor devices like integrated circuits (ICs) and large-scale integrated circuits (LSIs); liquid crystal devices (LCDs); image-pickup devices like charge-coupled devices (CCDs); and magnetic heads.
2. Description of the Related Art
Devices, such as for example, semiconductor devices like ICs and LSIs; LCDs; image-pickup devices like CCDs; and magnetic heads are manufactured by photolithography. In the photolithography, a pattern of a reticle (also called an original or a mask) is projected onto a photosensitive substrate (also called a substrate) such as a semiconductor wafer at a predetermined magnification, so that the photosensitive substrate is exposed to light. With an exposure apparatus of a related art, a projection optical system and a wafer have a gap therebetween that is filled with air or gas such as nitrogen. Thus the numerical aperture (NA) of the apparatus is smaller than 1.0.
Generally, a photosensitive material applied on the wafer has a suitable amount of exposure. Due to this, an optical member (for example, a half mirror) is disposed in an illumination optical system of the projection-type exposure apparatus. The optical member splits the exposure light and a light-receiving element (a first detector) such as a photodetector detects a light quantity of the split exposure light. On the basis of the output from the light-receiving element, the amount of exposure is controlled in order to expose the material to the light with the suitable amount of exposure.
In addition, light transmittances of the illumination optical system and the projection optical system may be slightly varied with time. Therefore, the light-receiving element is necessary to be calibrated based on an illuminance at the surface of the wafer. To calibrate the first detector, a light-receiving element (a second detector) such as a photodetector mounted on a wafer stage measures the exposure light transmitted through the illumination optical system and the projection optical system, with respect to each of various exposure conditions at a position near an image plane of the projection optical system.
In recent years, semiconductor devices are becoming miniaturized. Due to this, an excimer laser source, which emits far ultraviolet rays (vacuum ultraviolet rays), is being used as a light source of the projection-type exposure apparatus. However, if the excimer laser light is used as exposure light, it has been revealed that the optical properties of a glass member and a coating film of an optical component and the light-receiving sensitivity of a detection system such as a photodetector are gradually deteriorated. In addition, an incident angle of light relative to the light-receiving element on the wafer stage may be varied depending on the exposure conditions of the wafer, resulting in the sensitivity of the light-receiving element being varied depending on the incident angle.
To solve this, the light-receiving sensitivity of the second detector is calibrated by using an energy monitor (a third detector), which is detachably mounted on the wafer stage, and has a light-receiving sensitivity calibrated in predetermined gas such as air or nitrogen (with a relatively high accuracy as compared with that of the second detector). As described above, the amount of exposure may be controlled highly accurately (see Japanese Patent Laid-Open No. 2000-150334).
In the projection-type exposure apparatus having a numerical aperture that is smaller than 1 (NA<1.0), since a gap between the projection optical system and the wafer is filled with air or nitrogen, the above energy monitor is used for calibrating the light-receiving sensitivity of the light-receiving element. Meanwhile, in an immersion projection-type exposure apparatus that can have a numerical aperture of 1 or greater (NA≧1.0), the gap between the projection optical system and the wafer is filled with liquid (an immersion medium) such as pure water having a refractive index n that is greater than 1.0. Since the energy monitor for calibrating the light-receiving sensitivity presupposes detection of light incident from the gas, it is difficult to calibrate the sensitivity highly accurately upon detection of light incident from liquid.